Contrary to my previous belief, the British Air Passenger Duty is the same for all British airports. It's often mentioned here that "Heathrow" departure taxes can be very high; at least the APD portion is the same at LGW. I don't know if BAA adds their own hit on top of the government's - their web site is awful. The APD is described here in great detail:
http://customs.hmrc.gov.uk/channelsP...505#P200_11988

Interestingly, there's an example in which a pax leaves UK for Paris in coach, connects there to a J seat to the U.S., and is thus liable for the GBP40 departure tax out of the UK.